Hole-trap-compensated scintillator material and computed tomography machine containing the same

ABSTRACT

Afterglow in a luminescent material in which a significant factor in afterglow is the release of holes from hole traps in the scintillator material is substantially reduced by adding cerium as a hole-trapping species to the scintillator composition which successfully competes with the hole traps in the basic scintillator composition. In gadolinium gallium garnet activated with chromium, the addition of cerium in the range of 0.2 to 0.255 weight percent reduces afterglow in this manner while exhibiting excellent stability to x-ray damage after exposure to an initial pair of x-ray pulses.

RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.06/960,203, filed Oct. 13, 1992, now U.S. Pat. No. 5,318,722, which inturn is a continuation of application Ser. No. 07/546,824, filed Jun.29, 1990, now abandoned.

This application is related to application Ser. No. 07/547,007, filedJun. 29, 1990, now U.S. Pat. No. 5,057,692, issued Oct. 15, 1991,entitled "High Speed, Radiation Tolerant, CT Scintillator SystemEmploying Garnet Structure Scintillators" by C. D. Greskovich et al. andapplication Ser. No. 07/547,006, filed Jun. 29, 1990, entitled"Transparent Polycrystalline Garnets" by C. D. Greskovich et al., eachof which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of solid state scintillators,and more particularly, to the field of high speed solid statescintillators for computed tomography machines.

2. Background Information

A luminescent material absorbs energy in one portion of theelectromagnetic spectrum and emits energy in another portion of theelectromagnetic spectrum. A luminescent material in powder form iscommonly called a phosphor, while a luminescent material in the form ofa transparent solid body is commonly called a scintillator.

Most useful phosphors emit radiation in the visible portion of thespectrum in response to the absorption of the radiation which is outsidethe visible portion of the spectrum. Thus, the phosphor performs thefunction of converting electromagnetic radiation to which the human eyeis not sensitive into electromagnetic radiation to which the human eyeis sensitive. Most phosphors are responsive to more energetic portionsof the electromagnetic spectrum than the visible portion of thespectrum. Thus, there are powder phosphors which are responsive toultraviolet light (as in fluorescent lamps), electrons (as in cathoderay tubes) and x-rays (as in radiography).

Two broad classes of luminescent materials are recognized. These areself-activated luminescent materials and impurity-activated luminescentmaterials.

A self-activated luminescent material is one in which the purecrystalline host material upon absorption of a high energy photonelevates electrons to an excited state from which they return to a lowerenergy state by emitting a photon. Self-activated luminescent materialsnormally have a broad spectrum emission pattern because of therelatively wide range of energies which the electron may have in eitherthe excited or the lower energy states with the result that any givenexcited electron may emit a fairly wide range of energy during itstransition from its excited to its lower energy state, depending on theparticular energies it has before and after its emissive transition.

An impurity activated luminescent material is normally one in which anon-luminescent host material has been modified by inclusion of anactivator species which is present in the host material in a relativelylow concentration such as in the range from about 200 parts per million(ppm) to about 1,000 ppm. However, some phosphors require several molepercent of activator ions for optimized light output. With an impurityactivated luminescent material, the host crystal absorbs the incidentphoton and the absorbed energy may be accommodated by the activator ionsor it may be transferred by the lattice to the activator ions. One ormore electrons of the activator ions are raised to a more excited state.These electrons, in returning to their less excited state, emit a photonof luminescent light. In many commonly employed impurity activatedluminescent materials, the electrons which emit the luminescent lightare d or f shell electrons whose energy levels may be significantlyaffected or relatively unaffected, respectively, by the surroundingcrystal field. In those situations where the activator ion is not muchaffected by the local crystal field, the emitted luminescent light issubstantially characteristic of the activator ions rather than the hostmaterial and the luminescent spectrum comprises one or more relativelynarrow emission peaks. This contrasts with a self-activated luminescentmaterial's much broader emission spectrum. In those situations where theelectron energies of the activator ions are significantly affected bythe crystal structure, the luminescent spectrum is normally a fairlybroad one similar to that of a self-activated luminescent material. Thehost material of an impurity activated luminescent material normally hasmany other uses in which no activating species is present. In some ofthose uses, that host material may include other species to modify itsproperties, and may even include constituents which are luminescentactivators, but which are included in the composition because ofnon-luminescent characteristics which they impart to that composition.

There are a vast number of known phosphors each of which has its own setof properties such as the turn-on delay, efficiency, primary decay time,afterglow, hysteresis, luminescent spectrum, radiation damage and soforth. The turn-on delay of a luminescent material is the time periodbetween the initial impingement of stimulating radiation on theluminescent material and the luminescent output reaching its maximumvalue, for a constant intensity of stimulating radiation. The efficiencyof a luminescent material is the percentage of the energy of theabsorbed stimulating radiation which is emitted as luminescent light.When the stimulating radiation is terminated, the luminescent outputfrom a scintillator decreases in two stages. The first of these stagesis a rapid decay from the full luminescent output to a low, but normallynon-zero, value at which the slope of the decay changes to asubstantially slower decay rate. This low intensity, normally long decaytime luminescence, is known as afterglow and usually occurs withintensity values less than 2% of the full intensity value. The initial,rapid decay is known as the primary decay or primary speed and ismeasured from the time at which the stimulating radiation ceases to thetime at which the luminescent output falls to l/e of its full intensityvalue.

A luminescent material exhibits hysteresis if the amount of luminescentlight output for a given amount of incident stimulating radiationdepends upon the amount of stimulating radiation which has been recentlyabsorbed by the luminescent material. The luminescent spectrum of aluminescent material is the spectral characteristics of the luminescentlight which is emitted by that material.

Radiation damage is the characteristic of a luminescent material inwhich the quantity of light emitted by the luminescent material inresponse to a given intensity of stimulating radiation changes after thematerial has been exposed to a high radiation dose. Radiation damage maybe measured by first stimulating a luminescent material with a known,standard or reference, intensity of radiation. The initial output(I_(o)) of the photodetector in response to this reference intensity ofincident stimulating radiation is measured and recorded or stored. Next,the luminescent material is exposed to a high dosage of radiation.Finally, the luminescent material is immediately again exposed to thereference intensity of stimulating radiation and the final output(I_(f)) of its photodetector, in response to this reference intensity ofstimulating radiation, is measured and stored or recorded. The radiationdamage (RD) may then be 5 expressed as: ##EQU1## Ideally, the radiationdamage should be as small as possible. In most luminescent materials, itis a negative number because If is normally less than I_(o). However, ifthe afterglow magnitude is ≧0.1% at ˜100 milliseconds after cessation ofx-radiation, then unreliable and positive numbers for radiation damagemay be obtained.

In phosphors for use in radiography, many of these characteristics canvary over a wide range without adversely affecting overall systemperformance. In other applications, each of these characteristics mustbe strictly specified to obtain maximum or practical performance.

In a computed tomography (CT) scanning system, an x-ray source and anx-ray detector array are positioned on opposite sides of the subject androtated around the subject in fixed relation to each other. Early CTscanning systems employed xenon gas as their x-ray detection medium. Inthese systems, incident x-rays ionize the xenon gas and the resultingions are attracted to charged plates at the edge of the cell and thescintillator output is a charge or current. More recently, CT scannerswith solid scintillators have been introduced. In a solid scintillatorsystem, the scintillator material of a cell or element absorbs x-raysincident on that cell and emits light which is collected by aphotodetector for that cell. During data collection, each cell orelement of the detector array provides an output signal representativeof the present light intensity in that cell of the array. These outputsignals are processed to create an image of the subject in a mannerwhich is well known in the CT scanner art. It is desirable for theluminescent material in a CT scanner to have a linear characteristic inwhich the light output is a linear function of the amount of stimulatingradiation which is absorbed in order that light output may be directlyconverted to a corresponding intensity of stimulating radiation in alinear manner.

In systems such as CT scanners, the luminescent material must have manyspecialized characteristics which are not needed in many of thepreviously mentioned phosphor based systems. First, in x-ray based CTsystems, it is desirable to absorb substantially all of the incidentx-rays in the luminescent material in order to minimize the x-ray doseto which the patient must be exposed in order to obtain the computedtomography image. In order to collect substantially all of the incidentx-rays, the luminescent material must have a thickness in the directionof x-ray travel which is sufficient to stop substantially all of thex-rays. This thickness depends both on the energy of the x-rays and onthe x-ray stopping power of the luminescent material. Second, it isimportant that substantially all of the luminescent light be collectedby the photosensitive detector in order to maximize overall systemefficiency, the signal to noise ratio and the accuracy with which thequantity of incident stimulating radiation may be measured. In order toextract substantially all of the luminescent light generated in theluminescent material of the CT scanner, the luminescent material shouldbe transparent to the luminescent light. Otherwise much of theluminescent light will not reach the photosensitive detector because ofscattering and absorption within the luminescent material. Consequently,the luminescent material is provided in the form of a solid bar which issubstantially transparent to the luminescent light and which is thickenough in the direction of x-ray travel to absorb substantially all ofthe incident x-rays. This complicates both the selection of aluminescent material for use in CT scanning and its preparation sincemany materials which are known to luminesce and which have been used ortested as powder phosphors cannot be provided in the form of a solid barhaving the necessary transparency.

The luminescent properties of materials have not been tabulated inhandbooks in the manner in which the melting point, boiling point,density and other more mundane physical characteristics of variouscompounds have been tabulated. Most luminescent data is found inarticles with respect to particular materials which the authors havemeasured for one reason or another. Further, most characterization ofluminescent materials has been done using ultraviolet (UV) light as thestimulating radiation because ultraviolet light is more easily producedthan x-rays and is generally considered less harmful. Unfortunately,there are a number of materials which are luminescent in response toultraviolet light stimulation which are not luminescent in response tox-ray stimulation. Consequently, for many materials, even thatluminescent data which is available provides no assurance that thematerial will luminesce in response to x-ray stimulation. Further, formany applications of phosphors many of the parameters which must beclosely controlled in a scintillator for use in a state of-the-art CTscanning system are unimportant and thus have not been measured orreported. Consequently, existing luminescent material data provideslittle, if any, guidance in the search for a scintillator materialappropriate for use in a state-of-the-art CT scanning system. Among theparameters on which data is generally unavailable are radiation damagein response to x-ray stimulation, afterglow, susceptibility toproduction in single crystalline form, hysteresis phenomena, mechanicalquality and in many cases, even whether they are x-ray luminescent. Thelarge number of parameters which must meet strict specifications inorder for a given material to be suitable for use in a state-of-the-artCT scanner, including the ability to provide the material in the form oftransparent scintillator bodies, makes the process of identifying asuitable scintillator material one which essentially begins from scratchand is akin to searching for "a needle in a haystack". The difficulty ofidentifying such a material is exemplified by the use of cadmiumtungstate and cesium iodide activated with thallium in CT scanningmachines presently being marketed despite the fact that each of thesematerials has a number of characteristics (discussed below) which areconsidered undesirable for a state-of-the-art CT scanner scintillator.

There are several reasons that it is desirable that the radiation damagebe as small as possible. One disadvantage of high radiation damage isthat as radiation damage accumulates, the sensitivity of the systemdecreases because of the progressively smaller quantity of light whichis emitted by the scintillator material for a given stimulating dosageof radiation. Another disadvantage is that for too high a radiationdamage, the scintillation detectors must eventually be replaced becauseof the cumulative effects of the radiation damage. This results in asubstantial capital cost for the replacement of the scintillationdetecting system. A more bothersome, and potentially even more expensiveeffect of high radiation damage is a need to recalibrate the systemfrequently during the working day, and potentially as frequently asafter every patient. Such recalibration takes time and also exposes thescintillator material to additional radiation which contributes furtherdamage. It is considered desirable that the radiation damage of ascintillator material for use in a CT scanning system be small enoughthat calibration of the system at the beginning of each working day issufficient to ensure accurate results throughout the working day.

One way of providing the luminescent material in the form of atransparent bar is to employ a single crystalline luminescent materialwhich is transparent to its own luminescent radiation. A common methodof growing single crystals is the Czochralski growth technique in whichappropriate source materials are placed in a high temperature cruciblewhich is often made of iridium (Ir) and the crucible and its contentsare heated to above the melting point of the desired single crystallinematerial. The resulting molten material is known as the melt. Duringgrowth, the melt temperature is held at a value at which the upperportion of the melt is cool enough for single crystalline material togrow on a seed crystal brought into contact with the melt, but not tospontaneously nucleate. A seed crystal of the desired material or one onwhich the desired material will grow as a single crystal is lowered intocontact with the top of the melt. As the desired crystalline materialgrows on the seed crystal, the seed crystal is withdrawn (pulled upward)at a rate which maintains the growing boule of single crystallinematerial at a desired diameter. Typically, the seed crystal is rotatedduring growth to enhance the uniformity of the growing crystal. Thesource material which is initially placed in the crucible may take anyappropriate form, but is normally a mixture of appropriate quantities ofsource materials which together provide a melt having the stoichiometerydesired for the single crystalline material to be grown.

When a pure crystal is grown from a corresponding melt, the Czochralskigrowth technique normally provides a high quality, uniform compositionsingle crystal of the desired composition. When it is desired to producea crystal having substitutions for some portion of the atoms of the purecrystalline material, the growth dynamics are more complex and themanner in which the substituent enters into the crystal structure andthus its concentration in the melt and boule as functions of time dependon a number of characteristics. One of the effects of thesecharacteristics is characterized as the segregation coefficient. Thesegregation coefficient has a value of 1 when the substituent isnormally present in the solid boule in the same ratio as it is presentin the source melt. The segregation coefficient is greater than 1 whenthe substituent is normally present in the solid boule in greaterconcentration than it is present in the source melt and the segregationcoefficient is less than 1 when the substituent is normally present inthe solid boule in lesser concentrations than it is present in the melt.While there are a number of different fundamental reasons for thesedifferences, the segregation coefficient is an effective means ofexpressing the result.

Where slabs or bars of the single crystalline material are desired, theCzochralski-grown single crystalline boule is sliced into wafers andthen into bars of the desired configuration. The only two singlecrystalline luminescent materials known to be in use in commercial CTscanning systems are cesium iodide (CsI) and cadmium tungstate (CdWO₄).The cesium iodide is thallium (Tl) activated while the cadmium tungstateis a pure, self-activated luminescent material. CsI produces aluminescence output having a peak emission at about 550 nm and exhibitsappreciable hysteresis and radiation damage. CdWO₄ produces aluminescence output having a peak at about 540 nm and exhibits highradiation damage, although to a lesser extent than CsI. The radiationdamage with CsI is severe enough, that recalibration of the systembetween patients is often desirable. While the radiation damage in CdWO₄is less than that, recalibration more than once a day is considereddesirable. As a consequence of these radiation damage characteristics,systems which employ either of these materials as their scintillatingmaterial suffer from a decrease in sensitivity as radiation damageaccumulates and must eventually have their scintillator system replaced.

In a CT scanning system, one of the crucial characteristics of ascintillator bar is its Z-axis response curves. Individual scintillatorbars are normally narrow for maximum resolution and deeper than wide toprovide adequate x-ray stopping power and relatively long perpendicularto the plane of the x-ray beam/scintillator system in order to collectsufficient x-rays to be efficient. The Z-axis characteristic is thephotodetector output in response to a constant intensity, narrow, x-raystimulating beam as that beam is scanned from one Z-direction end of thescintillator bar to the other. Ideally, this characteristic is symmetricabout the longitudinal center of the scintillator bar and increasesmonotonically from each end to the center. The increase in output nearthe ends of the bar is preferably complete once the entire Z-directionthickness of the beam is disposed on the scintillator bar, with theoutput being substantially uniform along the intervening portion of thebar.

In order to meet these Z-axis requirements, the scintillator bar musthave substantially uniform optical, luminescent and source radiationabsorption properties along its entire length. For single crystal,impurity-activated scintillator bars, this requires the ability to growsource boules having uniform luminescent activator concentration bothradially and lengthwise of the boule, since the luminescent output isdependent on the local concentration of the activator ion. Consequently,the process of selecting a scintillator material for a CT scanner, inaddition to determining all of the other important properties of thematerial, must also include establishing the feasibility of producingscintillator bars with acceptable Z-axis characteristics.

In a CT scanner, it is preferable to provide a reflective surface on allsurfaces of the scintillator bar other than the surface along which thephotodetector diode is disposed. Thus, a typical solid scintillationdetector system comprises a plurality of individual scintillator barspositioned side-by-side with an individual photodetector diode coupledto each scintillator bar to convert its luminescent light into acorresponding electrical signal. It is important in such a system thatall of the scintillator bars have similar overall conversionefficiencies (that is, substantially identical electrical output signalsfor identical incident x-ray radiation). This places another limitationon the selection of the scintillator material in that it must bepossible to produce a sufficient quantity of scintillator bars havingsimilar characteristics to assemble a scintillation detector having asmany as 1,000 or more elements.

The primary decay time determines how fast a CT scanner can scan apatient since it is necessary for the luminescent output in response toradiation incident in one position of the scanner to have ceased beforethe luminescent output at another position of the scanner can beaccurately measured. At present, a primary decay time of less than 500microseconds is preferred, with still lower values being more desirableif they can be obtained without undesirable affects on other propertiesof the scintillator material such as maximum light output, radiationdamage and hysteresis. It is also desirable that the maximum afterglowlevel be very small and that it decay relatively rapidly. For modern CTscanners, afterglow may be measured at 100 to 150 milliseconds afterstimulating radiation termination and again at 300 milliseconds tocharacterize a scintillator material. An afterglow of less than 0.1% isconsidered highly desirable since the photodetector cannot distinguishbetween luminescent light which is a result of afterglow from earlierstimulation and luminescent light which is a result of presentstimulation. Thus, afterglow can limit the intensity resolution of a CTscanner system.

For purposes of comparing the efficiency of different candidatescintillator materials, it is convenient to normalize light output. Theamplitude of the output signal from a photodetector diode in response tostimulation of a standard sized scintillator bar of the candidatematerial with an established reference intensity of x-rays is comparedwith the output produced by cadmium tungstate of the same configurationin response to the same stimulation. Cadmium tungstate is a convenientstandard because the self-activated nature of its luminescence resultsin substantially fixed light output for a given intensity of stimulatingradiation so long as it has not been heavily radiation damaged, sinceits light output does not depend on the concentration of an activator.Thus, light output data taken by different individuals and at differenttimes can be directly compared without having to first establish thecalibration of different test setups.

It is desirable to have computed tomography scanning systems operate asfast as possible to maximize the number of patients which can beexamined by a computed tomography scanner each working day and becausethe shorter time a scan takes, the easier it is for a patient to holdstill during the scan. Further, the movement of internal organs isminimized.

As the scanning speed of a CT system is increased, the signal amplitudedecreases for a fixed x-ray dose rate. Consequently, the signal-to-noiseratio, the contrast and thus the useful intensity resolution willdecrease unless system parameters are adjusted to reduce noise. In orderto reduce noise, the primary decay time of the scintillator should bereduced to a value where it does not contribute noise to the system. Theafterglow should also be reduced as much as possible, since it providesa background luminescence intensity which is a noise contribution to thephotodetector output. Selecting a scintillator material having its peakoutput in the vicinity of the peak sensitivity of the photodetector hasthe effect of reducing noise by increasing signal amplitude. Othermodifications can also assist in maintaining the signal-to-noise ratio.

As the CT scanner field has matured, the speed of the electronics hasincreased, thus making faster scintillators desirable in order that adata scan may be performed in less time. It is now desired to operate CTscanning systems at speeds which require scintillators which are muchfaster than what was required as little as five years ago. Consequently,there is a vast lack of data about known solid luminescent materialswhich would be needed in order to select and make a scintillatormaterial which is appropriate for use in a state-of-the-art CT scanningsystem where high speed electronics must be matched by a still higherspeed scintillation material.

Separate from the problem of determining all these characteristics forindividual candidate materials, is the problem that in a scintillationscanner, material must be provided in the form of a transparent solidbody. Many luminescent materials which can be provided in powder formcannot be provided in a single crystalline form and thus are notavailable as transparent bodies. This inability to produce particularluminescent materials as single crystalline material can be a result ofincompatibility of crystal structures, instability at Czochralski growthtemperatures, low solubility of some components of a luminescentmaterial in the crystal structure or the melt, a segregation coefficientwhich results in a non-uniform distribution within the boule of theadditives and/or substituent or other reasons. Consequently, even if aparticular luminescent composition is identified as apparently havingdesirable properties for use in a scintillation detector of a computedtomography machine, production of such a scintillator detector is notstraightforward. In many cases, the desired composition cannot beproduced as a single crystalline material.

Scintillation counters are used to count high energy particles, inphysics research. These scintillation counters normally comprise a solidtransparent body (often a plastic with a luminescent material dispersedin it) which is coupled to a photomultiplier tube to detect the veryfaint luminescence produced by absorption of a single particle. Thematerials used for such scintillation counters must have a very shortprimary decay time (preferably much less than 100 nanoseconds) in orderto distinguish separate, but closely spaced-in-time events from eachother in order that the desired counting may take place. The othercharacteristics which are important to the use of a material as thescintillator in a CT scanning system are of little consequence in thescintillation counter art so long as the afterglow is low enough that anew primary scintillation can be distinguished from any backgroundafterglow resulting from previous events. These scintillation counterscan use luminescent materials whose afterglow would present a problem inthe CT scanning art. Consequently, although work has been done onscintillation materials for use in scintillation counting applications,such work is only peripherally relevant to a search for a scintillationmaterial for use in a CT scanning system.

There are a number of luminescent materials which can be produced byflux growth techniques as small single crystals, but which cannot beproduced as large single crystals because they are unstable at hightemperatures and decompose into constituent materials. Other luminescentmaterials have been produced as thin films in attempts to developphosphors for projection cathode ray tubes in order to minimize lightloss due to scattering in amorphous or polycrystalline films. Suchmaterials have no utility for the scintillators of CT scanners in theabsence of an ability to provide a transparent body having sufficientthickness (generally at least 1 mm thick) for the material to beeffective at stopping the x-rays employed in a CT scanning system.Further, the reports of the development work done on these materialscontain no data on many characteristics which are crucial to determiningwhether a material is suitable for use in a CT scanning system.

A polycrystalline alternative to the single crystalline scintillatormaterials cesium iodide and cadmium tungstate is disclosed in U.S. Pat.Nos. 4,421,671; 4,466,929; 4,466,930; 4,473,413; 4,518,545; 4,518,546;4,525,628; 4,571,312; 4,747,973 and 4,783,596. The scintillatorcomposition disclosed in these patents is a cubic yttrium gadoliniumoxide doped with various rare earth elements to provide a scintillatormaterial having desired luminescent properties. These materials have notbeen prepared in single crystalline form because of the difficulty ofgrowing crystals with desired, uniform distribution of all of thenecessary constituents. As is further disclosed in the above recitedpatents, a method was developed for providing this dopedyttrium-gadolinium oxide scintillator material in a polycrystallineceramic form in which it is sufficiently transparent to provide anexcellent scintillator material. This material has the substantialadvantage over the cesium iodide and cadmium tungstate of beingessentially free of radiation damage and hysteresis as well as having asufficiently low afterglow to satisfy the requirements for a highquality CT scanning system. Unfortunately, this material has a primarydecay time on the order of 1,000 microseconds and this is not as fast asis desired for present state-of-the-art CT scanning systems.

German patent DE 37 04 813 A1 describes a single crystal Gd_(3-x) Ce_(x)Al_(5-Y) Sc_(Y) O₁₂ scintillator prepared either by first spray drying asource sulfate solution and calcining the dried sulfate or mixingoxides--each followed by pressing, sintering, melting and pulling asingle crystal in a high vacuum. A spectrum for the luminescent outputfrom this material is also presented with its peak in the vicinity of560 nm.

It would be desirable to have a scintillator which is fast, has a lowafterglow, no hysteresis, no non-linearity in output, high x-raystopping power, high light output for a given stimulating x-ray inputand which emits light at a frequency where photodetector diodes areparticularly sensitive.

Single crystalline yttrium aluminum garnet (YAG) doped with neodymium isa known laser material. This material has also been further doped withchromium to increase the absorbence of the light frequency used tooptically pump a YAG laser. While attempts have been made to producetransparent polycrystalline YAG, such attempts have not been successful,see for example, "Translucent Y₃ Al₅ O₁₂ Ceramics", G. de With et al.,Materials Research Bulletin, Vol. 19, p. 1669-1674, 1984. Reducedopacity or increased translucency or transparency has been reported insintered YAG where magnesium oxide or silicon dioxide was included inthe composition in a concentration of 500-2,000 ppm. However, even withthis addition, true transparency is not obtained. Further, the inclusionof such transparency promoters in a scintillator material would beundesirable because of the potential for these impurities to adverselyeffect one or more of the desirable properties of a scintillatormaterial.

Many garnets are transparent in the infrared region. Consequently,transparent ceramic garnets would be desirable for use as combinedvisible/infrared windows where true transparency was obtainedthoroughout this portion of the spectrum.

The particular compositions discussed in the two related applicationsSer. No. 07/547,007, now U.S. Pat. No. 5,057,692, "High Speed, RadiationTolerant, CT Scintillator System Employing Garnet StructureScintillators" and Ser. No. 07/547,006, "Transparent PolycrystallineGarnets" in general have desirable characteristics for luminescentmaterials for use in CT scanning and other short response time systems,but exhibit afterglow which is greater than desired. PG,22

It is known in the luminescent scintillator art that afterglow can beaffected by impurities present in the scintillator composition. In somecases, afterglow is increased by the presence of impurities, and inother cases, afterglow is decreased by the presence of impurities. Itwould be desirable to be able to predict what effect a particularimpurity would have on afterglow. However, the particular mechanismswhich control afterglow have not been well understood with the resultthat afterglow reduction in a particular luminescent material has been atrial and error process of adding selected impurities to the luminescentcomposition and then measuring the resultant effect. Thus, the goal ofpredictability has eluded the art even though trial and errorexperiments have in some cases determined both the utility of aparticular impurity or combination of impurities for afterglow reductionpurposes and the quantity or concentration in which that impurity shouldbe introduced into the host composition in order to have a desirableafterglow reduction effect without significant adverse effects on otherimportant properties of the luminescent material for the particularintended use.

With single crystalline luminescent materials of the general type inwhich a host crystalline composition is non-luminescent and to which aluminescent activator is added, there is a significant problem withdetermining what impurities are present, and, more particularly, inattempting to selectively introduce additional impurities, to determinewhether they have a beneficial effect on the luminescent properties ofthat material. The addition problem, as explained in related applicationSer. No. 07/547,006, "Transparent Polycrystalline Garnets", is acutebecause of the difficulty of independently controlling the quantity of aluminescent activator and a selected additional additive in a singlecrystalline material which is grown by the Czochralski growth technique.Consequently, the field of controlling the luminescent material'sresponse characteristics by the addition of additional dopants oradditives has not been a fruitful area for research with singlecrystalline luminescent materials. With luminescent materials intendedfor use as scintillators, the requirement for uniform transparency andcomposition has been a substantial stumbling block to the developmentand testing of multiple additive luminescent compositions.

As discussed in related application Ser. No. 07/547,006, "TransparentPolycrystalline Garnets", we have developed a technique for producingtransparent polycrystalline garnet scintillator bodies in which thecomposition can be closely controlled as a result of the method ofpreparation. This opens up the possibility of extensive trial and errortesting of different additives to see whether they may have a beneficialeffect on the luminescent properties of the desired composition.

Accordingly, there is a need for a better understanding of bothafterglow production and afterglow suppression mechanisms withinluminescent materials in order to facilitate the design and testing ofluminescent compositions and for luminescent compositions which exhibitreduced afterglow without substantial deterioration of other luminescentproperties.

OBJECTS OF THE INVENTION

Accordingly, a primary object of the present invention is to provide anunderstanding of afterglow creation mechanism which facilitatespredictable afterglow reduction.

Another object of the present invention is to significantly reduceafterglow in garnet luminescent materials without introducing asignificant adverse effect on other luminescent properties of the garnetmaterial.

Another object of the present invention is to reduce afterglow in achromium doped gadolinium gallium garnet luminescent material employedas a scintillator for a computed tomography machine.

Another object of the present invention is to reduce afterglow in achromium doped gadolinium or gallium based garnet luminescent materialemployed as a scintillator for a computed tomography machine.

Another object of the present invention is to reduce radiation damageessentially to zero in a chromium doped gadolinium gallium garnetluminescent material employed as a scintillator for a computedtomography machine.

SUMMARY OF THE INVENTION

The above and other objects which will become apparent from thespecification as a whole, including the drawings, are accomplished inaccordance with the present invention with an enhanced scintillatorhaving a basic crystalline scintillator composition which has a garnetstructure and exhibits afterglow which is at least partially a result ofradiative recombination of holes which are released from traps after thecessation of stimulation to which a hole trapping species has beenadded, in order to counteract the afterglow-inducing effect of holetraps in the basic scintillator composition.

This is particularly applicable to impurity-activated garnetcompositions which exhibit hole-trapping induced afterglow. Garnets ofinterest include gadolinium gallium garnet (Gd₃ Ga₅ O₁₂), gadoliniumscandium gallium garnet (Gd₃ Sc₂ Ga₅ O₁₂), gadolinium scandium aluminumgarnet (Gd₃ Sc₂ Al₃ O₁₂), each activated with chromium 3+ ions in aconcentration from about 0.07 to 0.2 wt % Cr₂ O₃ yttrium aluminum garnet(Y₃ Al₅ O₁₂) activated with cerium 3+ ions at a concentration of about0.33 wt % Ce₂ O₃ or neodymium 3+ ions at a concentration of about 0.85wt % Nd₂ O₃ are particular examples of such scintillator compositions.For simplicity, we shall denote Gd₃ Ga₅ O₁₂ as GGG, Gd₃ Sc₂ Ga₃ O₁₂ asGSGG, Gd₃ Sc₂ Al₃ O₁₂ as GSAG and Y₃ Al₅ O₁₂ as YAG. The most usefulcomposition range for these materials as transparent scintillators istheir solid solution garnet structure range of composition of thenominal compositions and includes partial substitutions which do notadversely affect their luminescent properties.

Each of these garnets exhibits relatively low afterglow, but would bemore suitable for some applications such as fast CT scanners if itexhibited lower afterglow. We have found that in Cr³⁺ activatedgadolinium gallium garnet, the addition of up to 0.10 or more weightpercent cerium in the form of an oxide reduces afterglow by as much as97% with light output reductions in the best cases of as little as 20%,while terbium and praseodymium additions reduce afterglow by factors ofup to 10 and almost 3, respectively with light output reductions of lessthan 25%.

The afterglow reduction is accompanied by a significant reduction inradiation damage.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings in which:

FIGS. 1 and 2 are tables (Tables 1 and 2) illustrating light output,pump-up, afterglow, radiation damage and primary speed of theluminescence for a number of GGG compositions as a function of theircomposition;

FIGS. 1A and 2A are tables (Tables 1A and 2A) which present the samedata as FIGS. 1 and 2, respectively, but with normalized values;

FIG. 1B is a table (Table 1B) illustrating light output, pump-up,afterglow and radiation damage (including repeat damage) for a specificGGG composition as a function of the presence of varying amounts ofcerium additive; and

FIG. 3 is a stylized perspective illustration of a portion of a CTmachine.

DETAILED DESCRIPTION

A computed tomography (CT) scanning system 100 is illustratedschematically in FIG. 3. This CT scanning system 100 comprises acylindrical enclosure 110 in which the patient or object to be scannedis positioned. A gantry 112 surrounds the cylinder 110 and is configuredfor rotation about the cylinder's axis. The gantry 112 may be designedto revolve for one full revolution and then return or may be designedfor continuous rotation, depending on the system used to connect theelectronics on the gantry to the rest of the system. The electronics onthe gantry include an x-ray source 114 which preferably produces a fanx-ray beam which encompasses a scintillation detector system 116 mountedon the gantry on the opposite side of the cylinder 110. The fan patternof the x-ray source is disposed in the plane defined by the x-ray sourceand the scintillation detector system 116. The scintillation detectorsystem 116 is very narrow or thin in the direction perpendicular to theplane of the x-ray fan beam. Each cell 118 of the scintillation detectorsystem incorporates a solid transparent bar of scintillator material anda photodetector diode optically coupled to that scintillator bar. Theoutput from each photodetector diode is connected to an operationalamplifier which is mounted on the gantry. The output from eachoperational amplifier is connected either by individual wires 120 or byother electronics to the main control system 150 for the computedtomography system. In the illustrated embodiment, power for the x-raysource and signals from the scintillation detector are carried to themain control system 150 by a cable 130. The use of the cable 130generally limits the gantry to a single full revolution before returningto its original position. Alternatively, slip rings or optical or radiotransmission may be used to connect the gantry electronics to the maincontrol system 150 where continuous rotation of the gantry is desired.In CT scanning systems of this type, the scintillator material is usedto convert incident x-rays to luminescent light which is detected by thephotodetector diode and thereby converted to an electrical signal as ameans of converting the incident x-rays to electrical signals which maybe processed for image extraction and other purposes. At present, one ofthe limitations on the capabilities of such systems is thecharacteristics of the scintillator compositions, whether they be xenongas or bars of solid scintillator material.

A class of luminescent materials which are appropriate for use asscintillators in high speed x-ray CT scanning systems of the typeillustrated in FIG. 3 has been identified in the related applicationSer. No. 07/547,007, now U.S. Pat. No. 5,057,692, "High Speed, RadiationTolerant, CT Scintillator System Employing Garnet StructureScintillators". In particular, in single crystalline form, theyluminesce in response to x-ray stimulation, have primary decay times ofless than 500 microseconds and have afterglow levels which vary withposition in the boule and range from more than 1% to about 0.1% at 100to 300 milliseconds after the cessation of x-ray stimulating radiation,exhibit radiation damage having a magnitude of less than 5% after anexposure to between 500 and 1,000 rads of ˜140kV x-rays, exhibitessentially no hysteresis and when grown as single crystals by theCzochralski technique, are reasonably transparent to their luminescentlight and typically have light outputs which range from about 100% toabout 350% of that produced by cadmium tungstate single crystal, amaterial used in commercial x-ray body scanners.

Preparation of these materials in polycrystalline form is disclosed inrelated application Ser. No. 07/547,006 entitled "TransparentPolycrystalline Garnets".

This class of scintillator material is based on impurity activatedluminescence of cubic garnet crystals. Garnets are a class of materialswith the crystal chemical formula A₃ B₅ O₁₂ in which the A cations areeight-coordinated with oxygens and the B cations are either octahedrally(six) or tetrahedrally (four) coordinated with oxygens. The crystalstructure is cubic with 160 ions per unit cell containing eight formulaunits. In accordance with the present invention, the A cations are rareearth or yttrium ions alone, in combinations and/or with activatorsubstitutions. The B cations may be rare earth ions or other ions,again, alone, in combinations and/or with substitutions. In particular,we have found that with activator ions substituted in theeight-coordinated or six-coordinated sites, these garnets areluminescent in response to x-ray stimulation. A particularly importantactivator ion which emits x-ray excited optical luminescence in thishost material is the chromium 3+ ion located in six-coordinated sites.

As discussed above, a number of the garnet luminescent scintillatormaterials disclosed in the related applications exhibit promisingluminescent properties for use as high speed scintillator materials.However, their afterglow is higher than is considered optimum. Inaccordance with prior art practice, we could have attempted to improvethe afterglow of these materials by a trial and error process of addingimpurities (which in this situation would be called afterglow reducers)in a trial and error attempt to find additives which reduce theafterglow of these materials without substantially worsening othercharacteristics of the luminescent scintillator material. However, sucha trial and error process is both expensive and time consuming where anumber of luminescent materials are of interest. Consequently, weundertook to understand the process involved in the afterglow in orderto devise systematic means of reducing the afterglow in a predictablemanner and thereby avoiding the time and expense of extensive trial anderror testing.

When a high energy photon such as x-rays or particles such as neutronsor alpha particles are absorbed in a scintillator, that high energyphoton or particle ejects electrons from their atomic orbitals therebyforming hole-electron pairs in which a mobile electron and a mobile hole(positive charge due to the absence of an electron) which are separatedfrom each other and move independently from each other. Hole-electronpairs can recombine giving off energy within the bulk of the material,but in a luminescent scintillator more frequently recombine at activatorsites. Holes and electrons are each susceptible to being held atcrystalline defects or impurities which exhibit a favorable chargeconfiguration - these sites are known as traps, with hole traps holdingholes and electron traps holding electrons. Holes and electrons are eachreleased from traps after a time which depends on the thermallizationrate for that particular type of trap.

Crystalline yttrium aluminum garnet (YAG) contains host, hole-trapping,localized energy levels due to intrinsic and extrinsic defects and isweakly luminescent. We theorize that when doped with an individualtrivalent activator such as the rare earths cerium, europium, gadoliniumand terbium in an attempt to increase its luminescence, these intrinsicand extrinsic host defects remain substantially unchanged and thereforecompete with the activator ions for the high energy carriers whichprovide the energy for luminescence when they transfer their energy toan activator ion. Thus, these defects limit luminescent efficiency andnormally contribute to afterglow. This has been concluded from the studyof the temperature dependence of the luminescence intensity of theundoped and single rare earth activated YAG. In response to electronbeam excitation, YAG doped with Eu³⁺ (Eu³⁺ in YAG is an electron trap)has a good light output, but it also has a high afterglow. Theluminescence intensity of YAG doped with Ce increases linearly with Ceconcentration at low temperatures consistent with the assumption thatthe Ce³⁺ ions act as stable hole traps in direct competition with theYAG hole traps. The nature of the impurity potential that the rare earthimpurities introduce in YAG (electron attractive or hole attractivepotential) is influenced by the electronic configuration (redoxproperties) of the particular rare earth. For example, Eu³⁺ with theelectronic configuration 4_(f) ⁶ has one electron less than ahalf-filled shell and can relatively easily be reduced to Eu²⁺ byattracting an electron. Ce³⁺, on the other hand, has one electron morethan a completely filled shell and can attract a hole to form a Ce⁴⁺center or species. The fact that Pr also shows a tetravalent state(Pr⁴⁺) indicates that Pr³⁺ should also form a hole attractive center.The ratio [Ln³⁺ ]/[Ln⁴⁺ ] depends on the presence of aliovalentimpurities in the crystal as well as heat treatment atmosphere, time andtemperature. "Ln" in the previous sentence stands for any of theLanthanide series of the rare earth elements. Aliovalent impurities arethose impurities which exhibit a different valence than the element forwhich they are substituted.

Polycrystalline GGG:Cr as disclosed in application Ser. No. 07/547,006,"Transparent Polycrystalline Garnets", exhibits luminescent propertiessimilar to those in the worst portion of the single crystalline boule inapplication Ser. No. 07/547,007, now U.S. Pat. No. 5,057,692, "HighSpeed, Radiation Tolerant, CT Scintillator System Employing GarnetStructure Scintillators". This is believed to be because of increasedconcentration of host defects as a result of the GGG's polycrystallinestructure and preparation and processing history. This can be explainedon the basis that many of these host defects are hole traps whichcompete with the activators for holes generated within the scintillatormaterial during absorption of stimulating radiation, thereby limitingluminescent efficiency.

Because of the spectral characteristics of the light emitted by the GGGdoped with chromium during the period of stimulation and steady stateoutput and during the post-stimulation (afterglow) portions of theluminescent response of this material, we conclude that radiativehole-electron recombination at chromium sites is the source of theluminescent light in both situations. In the stimulating radiationintensity range of interest in CT scanning systems and most othersystems, the garnet host material does not approach hole trapsaturation. As a consequence, the chromium activator and hole trapswithin the host material compete for holes. On the basis of theafterglow characteristics of these materials, it appears that the holetraps in this material have relatively low thermallization rates. Thisdeduction was supported by a comparison of the pump-up characteristicsof this luminescent material, the steady-state emission of this materialand the afterglow characteristics of this material. Pump-up is thatcharacteristic of a luminescent material and a scintillator, inparticular, in which the luminescent light output rather than increasinginstantaneously to its steady-state level in response to a step functionin stimulating energy, jumps to an initial value and then increases overa period of time to that steady-state level. This period of time isknown as the pump-up period and could be of the order of seconds. Thatis, the light output upon exposure to a step function in stimulatingradiation, jumps to an initial light output value and then increases inan asymptotic manner to the final steady state light output value. Thedifference between the initial and final steady state light outputvalues as a percentage of the final steady state output value is knownas pump-up.

Pump-up can be explained as being due to (or related to) the timeconstant for hole-traps in the host material to be filled to asteady-state condition in which the percentage of traps occupied hasbecome essentially constant, although individual, unoccupied trapscontinue to attract holes, while occupied traps release holes inaccordance with the thermallization rate for those traps. Once a steadystate of hole trap occupancy has been reached, the light output remainsconstant until the stimulating radiation is removed.

When the stimulating radiation is turned off or the luminescent materialis shielded from that stimulating radiation, the light output decreasesrapidly in what is known as the primary decay of the luminescent output.This primary decay time is a reflection of the fact that holes(hole-electron pairs) in the luminescent material have a finite lifetimewith the result that those holes which are disposed in the valence bandin the scintillator material at the time that the stimulating radiationis turned off, have a finite decay time which is reflected as theprimary decay. For most luminescent materials, at the end of the primarydecay time, the slope of the luminescent light's decay becomessubstantially lower and the light output enters the phase known asafterglow.

After the termination of the stimulating radiation, holes continue to bereleased by (or to escape from) hole traps in the scintillator materialat a rate which is determined by thermallization rate for those traps.These escaped or released holes can radiatively recombine at chromiumsites, thereby emitting the light which constitutes afterglow. It willbe recognized that because of the absence of stimulating radiation, agreatly reduced number of holes are available to be trapped inunoccupied traps. As a consequence, trap occupancy declines to zero overa period of time, just as afterglow decays to zero over a period oftime.

In typical applications of scintillator materials, especially in therange of stimulating radiation to which they are exposed in typicaldiagnostic imaging systems, the host material hole trapping levels arefar from being saturated even during the steady state output interval.We concluded that this results in competition between radiativerecombination sites (for example, chromium in GGG) and hole-traps (inmaterials which exhibit hole-trap limited afterglow).

Consequently, we decided that in materials exhibiting hole-trap-limitedafterglow, introduction of larger trapping cross section, substantiallyfaster thermallization rate, hole-trapping species at sufficientconcentration should counteract the basic scintillator traps(characterized by relatively slow thermallization rate) and therebyreduce afterglow. These hole trapping species should have the valencestate of the element for which they substitute in the lattice of thescintillator material as one of their valence states. This avoids a needto include a compensating addition to maintain charge neutrality. Thecerium, terbium and praseodymium have this characteristic whensubstituted in a garnet structure, since each has a 3+ valence state.The existence of the 3+ valence state does not exclude the possibilitythat the added species may in fact be found in a 4+ state in thecrystalline structure under some conditions.

The introduction of such impurities provides an additional competitorfor holes in our new, modified scintillator material. The resultingcompetition should result in beneficial modification of thescintillator's properties, thereby providing an enhanced scintillatormaterial. If the hole-trapping species has a nearly similar or largerconcentration and a larger capture cross section than the inherent orbasic hole traps of the basic scintillator material, then an overallshielding effect should substantially reduce the capture rate for andthe occupancy of basic hole traps. The additional hole traps created bythe inclusion of the larger cross section, fast thermallization rate,hole-trapping species in the scintillator composition can liberate holeseither by recombination thereat or by release therefrom. Any releasedholes are then available to be captured (1) by basic hole traps, (2) byadded hole traps (3) by radiative recombination sites or (4) bynon-radiative recombination sites.

Radiative recombination at Cr³⁺ sites after hole release would beexpected to have no significant effect on the maximum light output,since once a steady-state was reached, the rate of hole generation bystimulating radiation and the rate of hole extinction by radiativerecombination should be equal. However, where hole non-radiativerecombination at the trapping sites occurs, the light output decreaseswith increasing concentration of the added hole-trapping species, sincethat hole-trapping species diverts holes from the radiativerecombination mechanism into a nonradiative recombination mechanism.

Another factor which can contribute to reduced light output as a resultof the addition of a hole-trapping species to the basic scintillatormaterial is the hole-trapping species being absorptive of theluminescent light frequency of the activator species in the basicscintillator composition. Consequently, for each hole-trapping species,there can be a trade-off between afterglow reduction and an associatedreduction in light output or adverse effects on other luminescentproperties. As will be observed from the data presented in the tables,afterglow can be reduced by about 97% with only about a 20% reduction inlight output by modifying the scintillator composition in accordancewith this inventive theory while also reducing radiation damage by asmuch as 97%.

An added hole-trapping species (hts) has a capture volume of influencewhich is related to the capture cross- section of the resulting holetrap. The number of basic scintillator traps which are occupied duringsteady state stimulation may be expressed by the following equation:

    n=n.sub.o e.sup.-VC.sbsp.hts

where the added hole-trapping species has a concentration C_(hts). Thus,the number of occupied basic scintillator traps decreases withincreasing concentration of the hole-trapping species and withincreasing volume of influence for an individual member of thathole-trapping species. The afterglow data presented in the tables inFIGS. 1, 1A, 1B. 2 and 2A support our theory. Tables 1, 1B and 2 presentthe actual measured values for the individual samples while Tables 1Aand 2A present the same data as Tables 1 and 2, respectively, butnormallized to each of the data values for the 0.31 wt % Cr₂ O₃concentration in the GGG without other intentional additives. Tables 1Aand 2A are presented to simplify comparison of the characteristics ofthe different samples.

We prepared a substantial number of samples in accordance with thisinvention using the process of related application Ser. No. 07/547,006,"Transparent Polycrystalline Garnets". For this sample preparationprocess, we chose the ammonium hydroxide process of that applicationrather than the ammonium oxalate process because the ammonium hydroxideprocess produces a 100% quantitative yield and thus avoided a need fordetailed compositional analysis of the samples. In production, eitherprocess may be used as may other processes as may be found desirable.

The source compounds were 99.99% or higher purity in order to minimizethe unknown/uncontrolled impurities present in the final compositionwhich can effect radiation damage, afterglow and luminescent efficiency.

We start by forming a hydrochloric acid solution of the desired cationsin appropriate quantities. By appropriate quantities, we mean relativeconcentrations which result in the final (preferably transparent) bodycontaining the desired relative proportions of cations.

One way of forming this source chloride solution is by dissolving thesource oxides of the desired cations in hot concentrated hydrochloricacid. For those situations where a closely controlled final garnetcomposition is desired, especially where the absence of unknownimpurities is considered desirable, use of source compounds which are of99.99% or higher purity is preferred. Naturally, the source cations maybe provided as chlorides rather than oxides, if desired. Other sourcecompounds may also be used.

Once the source materials have completely dissolved in the hotconcentrated hydrochloric acid, the resulting solution is cooled to roomtemperature. The resulting solution should be clear and free ofprecipitates and free of settling out of any of the source material. Inthe event that precipitation or settling out of source material occurs,the solution should be reheated, and additional hydrochloric acid addedto the solution so that upon cooling to room temperature again, noprecipitation or settling out occurs. That is, enough hydrochloric acidshould be used to ensure that the source materials are not present at orabove their solubility limit at room temperature.

Separately, an ammonium hydroxide solution is prepared by diluting 30%NH₄ OH with an equal volume of deionized water. This diluted NH₄ OH isthen added drop-wise to the clear chloride solution while stirringvigorously.

During the process of adding the ammonium hydroxide a gel-likeprecipitate forms. The ammonium hydroxide solution is added until the pHis in the range from 7.8 to 8.3. Once the pH is in that range,precipitation is complete. Since our work was directed to establishingthe characteristics of these materials, we dripped the ammoniumhydroxide solution into the chloride cation source solution rather thanjust pouring the two together in order to ensure that no chemicalinhomogeneity or separation of phases occurred during our preparationprocess which might have adversely affected our test samples. Thisdripping was accomplished at a rapid drip rate which wasnear-to-streamlike.

If desired, the precipitate may be water and/or alcohol washed beforeseparating the precipitate from the liquid. This is done by allowing theprecipitate to settle, pouring off or otherwise removing most of theliquid and adding the wash water or alcohol, allowing the precipitate tosettle again, and again removing the clear liquid. Where high purityand/or closely controlled composition of the final transparent garnet isdesired, the wash water should be high purity, deionized water and thealcohol should be of standard reagent grade purity. This washing processremoves excess ammonium hydroxide and reaction products such as ammoniumchloride from the precipitate. The precipitate is then separated fromthe wash solution by filtering, centrifuging or other appropriatetechniques. This precipitate is a multi-component precipitate having asubstantially uniform chemical composition. This wet precipitate isbelieved to be a complex ammonium gadolinium-gallium-chromium hydroxide(when preparing chromium activated GGG), however, the detailed chemicalstructure of this precipitate has not been exactly determined and doesnot need to be known for the success of this process. This precipitateis preferably dried, such as by oven drying at a temperature ofapproximately 110° C. for a day or by vacuum or air drying to produce afine dry powder.

This fine powder was then heated in air and held at 900° C. for one hourto thermally decompose the hydroxide thereby forming a crystallinegarnet powder. The thermal decomposition temperature can vary over awide range, such as from about 600° C. to 1000° or 1100° C. with a rangeof 750° C. to 900° C. being typical.

This crystalline garnet powder may be directly pressed to produce acompact for sintering. However, if a transparent final sintered body isdesired, it is preferred to mill this powder to reduce agglomerationprior to pressing it to form compacts. This milling may be done in aball mill using zirconia grinding media and a liquid vehicle such aswater, methyl alcohol or isopropyl alcohol. Ball milling times fromabout 4 to 24 hours are effective. Alternatively, a fluid energy (gas)mill or a jet mill may be used with air pressure settings of from about60 to about 100 psi.

Where chromium is present in the gadolinium gallium garnet, it ispreferably in a concentration equivalent to chromium oxide between 0.05and 1.0 wt. %, most preferably between 0.1 and 0.6 wt. %, of the overallscintillator composition. Where the hole trapping species is cerium, itis present in the gadolinium gallium garnet in a concentration betweenabout 0.2 wt. %, and 0.255 wt. %, and most preferably between 0.214 wt %and 0.255 wt. % of the overall scintillator composition. Where thehole-trapping species is terbium or praseodymium, it is present in thegadolinium gallium garnet in a concentration between 0.005 and 0.15weight percent of the overall scintillator composition.

EXAMPLE

A desired reference composition Gd₃ Ga₄.96 Cr₀.04 O₁₂ was prepared bydissolving 5.38 g of Gd₂ O₃, 4.59 g Ga₂ O₃ and 0.11 g CrCl₃.6H₂ O(equivalent to 0.031 g Cr₂ O₃) in 37.5 ml of concentrated HCl.

Separately, 86.0 cc of 30% NH₄ OH was diluted with an equal volume ofdeionized water. This diluted NH₄ OH was then added drop-wise to theclear chloride solution while stirring vigorously. During this processthe pH of the solution was monitored. The ammonium hydroxide solutionwas added until the pH was increased to 8.1. Once this pH was reached,precipitation was complete, but the precipitate was still suspended inthe liquid vehicle because of its fine character.

This suspension was then vacuum filtered to separate the precipitateusing medium filter paper. When most of the liquid was gone, but beforethe liquid level was allowed to reach the precipitate collected on thefilter paper, 1000 cc of methanol were added to wash the precipitate andthe filtering was allowed to proceed until "all" of the liquid had beenremoved.

The resulting wet precipitate was dried for 12 hours at 50° C. undervacuum. This dried precipitate was then heated in air and held at 900°C. for one hour to thermally decompose the hydroxide precipitate to formthe garnet phase.

Without having been milled, 1 gram of the resulting garnet powder wasdie pressed in a 15.9 mm diameter circular die at a pressure of 3,500psi followed by isostatic pressing at room temperature at 60,000 psi toform a compact which was then sintered in flowing pure oxygen at atemperature of 1,550° C. for 2 hours. After sintering, the sample wasabout 12 mm in diameter and about 1.4 mm thick.

All of the samples which provided data for Tables 1 and 2 (and 1A and2A), were prepared in this manner with the only difference being thestarting composition. The samples prepared for Table 1B were prepared inthe same manner except that they were die pressed into rectangularplates of dimensions 51 mm×25 mm×2 mm thick. After sintering the plateshad dimensions of 36 mm×18 mm×1 mm thick.

The cerium, terbium and praseodymium additives that are initiallyintroduced into the hydrochloric acid solution substitute in a garnetstructure primarily in the 3+ oxidation state and thus were not matchedwith any compensating elements. However, during the high temperaturesteps of the scintillator production process, some fraction of theseafterglow reducing impurities may change from their initial 3+ valenceto a 4+ valence. That fraction is unknown at this time and may vary withthe details of the fabrication process. Consequently, the presence of 4+ions of cerium, terbium or praseodymium may contribute to the beneficialeffect of this addition, may detract from it or may have no effect onit.

Pure, stoichiometric gadolinium gallium garnet is comprised of 53.7weight % Gd₂ O₃ and 46.3 weight % Ga₂ O₃. Chromium, when added to GGG asan activator, substitutes for Ga³⁺ in the Ga lattice sites in the garnetstructure because of their almost identical ˜0.62 Å ionic radii.Consequently, in the tables in the Figures the Ga₂ O₃ weight % isreduced from the pure stoichiometric GGG weight % by the Cr₂ O₃ weight%. When Cr³⁺ is the only substituent, the formula may be written Gd₃Ga_(5-y) Cr_(y) O₁₂, where Y represents the number of moles of Cr³⁺ in amole of the garnet.

Cerium, terbium and praseodymium 3+ ions were concluded to behole-trapping species with probable fast thermallization times. The holetrapping properties of Ce and Tb when introduced alone into yttriumaluminum garnets are discussed in an article "The Relationship BetweenConcentration and Efficiency in Rare Earth Activated Phosphors", by D.J. Robbins et al., J. Electrochemical Society, September 1979, p. 1556.We propose Pr³⁺ should behave in a similar way based on its redoxproperties. Consequently, these three potential hole-trapping specieswere selected for testing to confirm that in fact the intentionaladdition of hole trapping species into an activated garnet would reduceafterglow in these materials. As will be observed from the datapresented in the tables, each of these species in fact produces theexpected result of reduced afterglow, although with differing degrees ofafterglow reduction at a particular concentration.

When Ce³⁺ is substituted in the GGG, it substitutes for Gd³⁺ in thelattice because the ionic radius of Ce³⁺ (1.14 Å) is not greatlydifferent from the radius (1.06 Å) of Gd³⁺ ions in the eight coordinatedsites in GGG and because its radius is much larger than that of Ga³⁺(˜0.62 Å). Thus, when Ce³⁺ is present as a substituent in addition toCr³⁺, the formula may be written Gd_(3-x) Ce_(x) Ga_(5-Y) Cr_(Y) O₁₂˜where X represents the number of moles of Ce³⁺ in a mole of the garnet.The same is true of the two other hole-trapping species listed in tables(terbium and praseodymium) with the result that the Gd₂ O₃ weight % isreduced from the pure, stoichiometric GGG value by the weight % of thehole-trapping species which is present.

A range of GGG compositions are listed in Tables 1, 1A, 1B. 2 and 2A inFIGS. 1, 1A, 1B, 2 and 2A, respectively. The compositions and samplesare the same in Table 1A as in Table 1 and in Table 2A as in Table 2,with the actual measured values listed in Tables 1, 1B and 2 andrelative or normalized values listed in Tables 1A and 2A to simplifyinterpretation of the data. Each of these samples was prepared andprocessed in an identical manner by the above described process. Eachsample was in excess of 90% of theoretical density. Based on weight lossand lattice parameter studies of garnets, the final compositions areconcluded to be essentially as intended. All of the samples weremeasured in the same manner.

Light output was measured using a photodiode which is sensitive towavelengths in the 300 nm to 1,100 nm range. For light out measurementsthe stimulation was a 60 KV/5 ma/0.35 sec x-ray pulse. The measuredvalue is the mean value obtained.

Pumpup was measured with a 60 KV/5 ma/1.12 sec x-ray pulse. Light outputwas plotted as a function of time from 90% to 100%. The measured pumpupis the percentage rise beyond the primary rise.

Radiation damage noted in FIGS. 1, 1A, 2 and 2A was measured by exposingthe circular sample to two consecutive 120 KV/200 ma/4 sec pulses 10seconds apart for a total exposure of 480 RADs, while radiation damagenoted in FIG. 1B was measured by exposing the rectangular sample inthree separate periods, each containing two consecutive 120 KV/200 ma/4sec x-ray pulses 10 seconds apart, for three separate 480 RAD exposures(or a total of 1440 RADs). Light output loss (or gain) with respect tothe unexposed sample was determined through probe pulses (60 KV/5 ma/0.2sec) before and after the 480 RAD dose for the sample of Tables 1, 1A, 2and 2A and before and after each separate 480 RAD dose for the sample ofTable 1B. A probe pulse 35 seconds after x-ray off is used as the"after" data point for determining the light loss percentage and thusthe radiation damage.

Afterglow was measured using a 60 KV/50 ma/0.5 sec x-ray pulse. Lightoutput measurement begins about 100 milliseconds before the x-rays areturned off. The percentage output with respect to the signal with x-rayson is determined as a function of time. The value at 100 msecs afterx-rays off is used as the data point. A narrow (0.1 inch wide) x-raybeam is used to avoid saturation of the diode.

Primary speed is measured with a 120 KV/0.05 sec x-ray pulse. Lightmeasurement begins 0.005 sec before the x-rays are turned off. Lightoutput is plotted as a function of time. The elapsed time after x-rayoff at which the light output falls to 1/e (36.7%) of the ON lightoutput is the primary speed.

The measured values of scintillator properties are accurate to .sup.±0.05 V for light output (LO), .sup.± 0 05% for pump-up (PU), ±0.02% forafterglow (AFG) of the disks and ±0.001% of the plates, ±0.2% forradiation damage (RD) and ±5 μs for primary speed (PS).

To summarize the table data, pure GGG doped only with Cr³⁺, measuredwith an instrument of ±0.02% accuracy, had a relatively high afterglowof 1.0% for 0.31 wt % Cr₂ O₃ and 0.70% for 0.15 wt % Cr₂ O₃. Theseafterglows decreased significantly with additions of as little as 0.015wt % Ce₂ O₃, and by as much as 97% for compositions containing 0.31 wt %Cr₂ O₃ and 0.06 to 0.12 wt % Ce₂ O₃. As indicated in Table 1B, andmeasured with an instrument accurate to ±0.001% pure GGG doped only withCR³⁺ had an afterglow of 0.006% for 0.31 wt % Cr₂ O₃ which decreased toextremely low values (i.e., down to 0.001%) as the Ce concentration wasincreased at least to 0.255 wt %. It is noteworthy that afterglow washalved as the Ce concentration was increased from 0.214 wt % to 0.255 wt% . For terbium (Tb) and praseodymium (Pr) additions, the reductions inafterglow were less dramatic. The best of these samples is five timesbetter than the best of the single crystal samples produced inaccordance with application Ser. No. 07/547,007, now U.S. Pat. No.5,057,692, "High Speed, Radiation Tolerant, CT Scintillator SystemEmploying Garnet Structure Scintillators".

The luminescent light output generally decreased when Ce, Tb or Pr wasintentionally added to the Cr activated GGG, except in the case of therectangular plate samples. Changes in pump up tend to track changes inafterglow for the various hole-trapping centers introduced. Radiationdamage values also decreased substantially with increasing hole trappingadditions. For the rectangular disks (Table 1B), the x-ray radiationdamage (RD) subscripts 1, 2, and 3 signify the first, second and thirdperiod of 480 RADs radiation exposure, respectively. Table 1B shows thatthe RD₂ value drops unexpectedly and precipitously as Ce in thecomposition is increased from 0.214 wt % to 0.255 wt %, and that boththe RD₂ and RD₃ values are essentially zero for the compositioncontaining 0.255 wt % Ce, signifying excellent stability to x-ray damageafter only the first period of exposure to x-rays. This allows theinitial dose of radiation needed to fabricate a scintillator exhibitingessentially zero radiation damage to be reduced from two periods (RD₁and RD₂) to a single period (RD₁) as Ce in the composition is increasedtoward 0.255 wt %.

The samples which provided the data for the table entries were nottransparent, but rather were only opaque/translucent because we chose toomit the milling and hot isostatic pressing steps from the processdescribed in application Ser. No. 07/547,006, "TransparentPolycrystalline Garnets", in order to expedite sample preparation. Toconfirm the validity of this technique as a means of determining theutility of these additives for transparent scintillators of the typedisclosed in application Ser. No. 07/547,006, we processed, for Table 1,a sample having the composition 53.69 wt % Gd₂ O₃ +0.051 wt % Ce+45.94wt % Ga₂ O₃ +0.31 wt % Cr₂ O₃ using the full process, including millingand hot isostatic pressing as described in application Ser. No.07/547,006. That is, following the thermal decomposition of the driedprecipitate, the resulting garnet powder was milled in water for 24hours using zirconia grinding media to reduce agglomeration (so that allparticles are less than 5 microns in size). After the milled suspensionwas air dried for 24 hours at room temperature, the resulting powder wasdie pressed at a pressure of 3,500 psi followed by isostatic pressing atroom temperature at 60,000 psi to produce a disk-shaped compact forsintering as described above.

Following sintering as described above, the sintered disk was immersedin Gd₂ O₃ packing powder in a molybdenum crucible after which the loadedcrucible was inserted in a HIP furnace and heated at a rate of 25°C./minute up to 1,500° C. in 25,000 psi of argon pressure After a soaktime of one hour at 1,500° C. the furnace and the sample therein werecooled to room temperature and depressurized. After this sample wasremoved from the hot isostatic pressing furnace, it was given anoxidation treatment at 1,550° C. for two hours in flowing oxygen toremove the dark green cast created by the conditions in the hotisostatic pressing furnace.

This sample was transparent and exhibited measured values of LO=0.55 V,PU=0.1%, AFG=0.02%, RD=0.3%, PS=130 μs, all of which are within themeasurement error of the values obtained for the correspondingopaque/translucent sample which provided the table data for thiscomposition.

In our work with Cr³⁺ activated GGG with cerium as an addedhole-trapping species, we have not detected cerium emission lines in theluminescent output up to 850 nm. This leads us to conclude that thecerium hole-trapping species acts (1) solely as a storage site for holeswhich reduces the occupancy of traps in the basic scintillatorcomposition, (2) as a recombination site at which holes recombine by anonradiative process or (3) a combination of (1) and (2). It is possiblethat there is cerium emission at a very low intensity which we have notdetected. Even if such emission is present, its low level indicates thatit is not a significant recombination path in our improved scintillatormaterial.

With cerium as the only dopant or potential activator added to GGG, acerium concentration of 60 parts per million (ppm) results in weakluminescence in response to x-ray excitation. At 260 ppm there is littleor no luminescence and at 600 ppm luminescence is virtuallyundetectable. The absence of significant Cr³⁺ luminescence in thecerium/chromium co-doped GGG is consistent with this observation.

As a further confirmation of our hole-trap-limited afterglow theory, weselected an effective electron trapping species (europium) for additionto the basic scintillator composition with the expectation that additionof an electron trapping species would further separate electron-holepairs and exacerbate the problem of afterglow. In other words, thepresence of Eu³⁺ as an electron trap would promote the charge separationbetween electrons trapped at Eu³⁺ sites and holes trapped in the basicscintillator composition's traps. As can be seen from the europium data(final entry in the tables in FIGS. 2 and 2A), addition of europium didin fact produce the expected increase in afterglow. The afterglow witheuropium almost doubled as compared to the Cr³⁺ only sample while lightoutput decreased by 36%.

It will be understood that the individually identified garnets do notneed to be at their stochiometric composition, but may be a solidsolution with a composition anywhere in the garnet phase compositionrange at the processing temperatures so that a single phase garnetcrystalline structure results, and that this restriction on compositiononly applies where a transparent scintillator is desired. By the solidsolution composition range, we mean the range of compositions for whichthe garnet phase is stable as a single phase in accordance with thepublished phase diagram for the Gd₂ O₃ --Ga₂ O₃ system. Outside thatrange the body is not a single phase because of the presence of a secondphase which results in a phase mixture rather than a solid solution.

While specific compositions have been specified, it will be recognizedthat other non-detrimental substituents may be substituted for some ofan element without departing from the scope of the appended claims, solong as the overall scintillation properties are acceptable. As anexample, we know that when we mill powders with zirconia milling media,zirconia is introduced into the composition in amounts which we havemeasured as high as 894 ppm. It is not known whether this addedzirconium substitutes for one of the other elements or ends up in thestructure in an intersitial location or primarily at grain boundaries.This addition occurs without apparent adverse effect on thescintillator's characteristics as can be seen from a comparison of thetransparent sample's characteristics and the characteristics of the samecomposition sample presented in Table 1 (which, since it was not milleddoes not include this zirconium addition). Hafnium, measured at as highas 12 ppm can also be added without apparent adverse affect. It is alsopossible that the zirconium acts as a transparency promoter during thesintering process. Similarly, yttrium and aluminum measured at as highas 79 ppm and 39 ppm, respectively, can be partially substituted,respectively, for the gadolinium and the gallium. These concentrationswere measured by glow discharge mass spectrometry. The upper limit onconcentration which does not produce an adverse effect is unknown foreach of these substituents and other substituents may be includedwithout adverse effect. Thus, the limits of composition for the specificscintillators are defined by their primary constituents and theirluminescent properties rather than being strictly limited to specifiedcomposition since other elements can be added without adverselyaffecting the scintillator's luminescent properties. That is, there is awide range of actual compositions for which the luminescence is a resultof the presence of a Cr³⁺ co-doped garnet which is gadolinium andgallium based.

These enhanced scintillator materials are suitable for use with suchhigh energy stimulation as x-rays, nuclear radiation, and an electronbeam.

While the invention has been described in detail herein in accord withcertain preferred embodiments thereof, many modifications and changestherein may be effected by those skilled in the art. Accordingly, it isintended by the appended claims to cover all such modifications andchanges as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A crystalline scintillator composition consistingof:gadolinium gallium garnet having the formula Gd₃ Ga₅ O₁₂ ; chromiumin a concentration in said garnet equivalent to chromium oxide beingbetween 0.05 and 1.0 weight percent of the overall scintillatorcomposition which is effective to render said scintillator compositionluminescent in response to high-energy stimulating radiation consistingof x-rays; and a hole trapping species, cerium, in a concentration insaid garnet of about 0.2 to about 0.255 weight percent of the overallscintillator composition which is effective to reduce the afterglowlevel relative to the afterglow level of said scintillator compositionin the absence of said hole trapping species while allowing thescintillator to withstand said radiation with essentially no damageafter receiving an initial dose of said radiation; said chromium beingsubstituted in said formula for said Ga and said hole trapping speciesbeing substituted in said formula for said Gd.
 2. The scintillatorcomposition recited in claim 1 wherein said crystalline scintillatorcomposition is polycrystalline.
 3. The crystalline scintillatorcomposition of claim 2 wherein said scintillator composition istransparent.
 4. The scintillator composition recited in claim 1 whereinsaid concentration of cerium is between about 0.214 weight percent andabout 0.255 weight percent of the overall scintillator composition. 5.The scintillator composition recited in claim 1 wherein saidconcentration of cerium is about 0.255 weight percent of the overallscintillator composition and said initial dose of radiation totals about480 RADs.
 6. In a computed tomography machine of the type having a solidstate scintillator, the improvement comprising:the scintillator beinggadolinium gallium garnet having the formula Gd₃ Ga₅ O₁₂, chromium in aconcentration in said garnet equivalent to chromium oxide being between0.05 and 1.0 weight percent of the scintillator which is effective torender said scintillator luminescent in response to high energystimulating radiation consisting of x-rays produced by said machine, andcerium in a concentration in said garnet of between 0.2 and 0.255 weightpercent of the scintillator which is effective to reduce the afterglowlevel relative to the afterglow level in the absence of cerium and toenable said scintillator to exhibit essentially zero x-ray damage afterreceiving an initial dose of said x-rays, said chromium beingsubstituted in said formula for said Ga and said cerium beingsubstituted in said formula for said Gd.
 7. The computed tomographymachine recited in claim 6 wherein said weight percent of chromium isbetween 0.1 and 0.6.
 8. The computed tomography machine recited in claim6 wherein said scintillator is polycrystalline.
 9. The computedtomography machine recited in claim 8 wherein said scintillator istransparent.